Knowledge of pathogenetic mechanisms involved in the induction of EAE has been gained indirectly from studies in the whole animal. In vitro cell studies (reported last year) indicated that proliferation in culture of BP-sensitized cells in the presence of antigen correlates well with the disease state but this proliferation is not induced exclusively by the region of te molecule responsible for disease induction. These studies led to an important observation that culture with antigen in vitro markedly enhanced (approximately 100-fold) the ability of BP-sensitized cells to transfer EAE. With the small number of cultured cells required for transfer (0.5-1 x 10 to the 7th power) we have been able to fractionate the BP-cultured cells into an active and an inactive population.